Fall 2013, The College of Saint Rose

Lecture 20: Two-Dimensional Arrays
Date: Tuesday, December 3, 2013

Agenda

• Announcements
• Final exam in one week: 10:45 AM in our regular classroom.
• same ground rules as first two exams: closed book, closed notes, closed neighbors
• practice questions out
• we can review on Thursday
• Lecture 19 assignment recap
• Completing the "SalesGraph" example
• Two-dimensional arrays
• In-class Exercise 20 - (20 lecture assignment points) due before the end of class.

Your task is to add some functionality to the Matrix2D example and write code to test it in the main method there. The functionality you are to add:

• A method max that returns the largest number in the matrix (5 points)
• A destructive method scale that multiplies each entry in the matrix by a number (5 points)
• A non-destructive method multiply that works much like add but computes the matrix-matrix multiplication result. If you don't remember how to multiply matrices, see the "Matrix Product" section of the Wikipedia entry to Matrix multiplication. (10 points)

Please submit only the file Matrix2D.java in Submission Box under "InClass20" before you leave class.

Due at the start of class, Thursday, December 5.

Please submit answers to these questions in Submission Box under "LA20" by the start of our next class. We will discuss these questions at the start of class, so no late submissions are accepted. Please be sure that your name is clearly indicated in all submissions.

The popular Sudoku puzzle game consists of a board that has 9 rows and 9 columns of numbers ranging from 1-9. Suppose we are writing a class to store a Sudoku board. We will assume that filled-in entries contain a number 1 through 9, and blank entries are represented by 0.

• Write a declaration of the instance variable board that would store the board using a two-dimensional array of int. Show also the construction of the array. (3 points)
• Write a method checkRow that takes a row number (let's use rows numbered 0-8 for simplicity) and checks that no two filled-in entries contain the same number within that row (this would be illegal in the game). It should return true if the row is legal, false if not. (6 points)
• Assume you have a similar method checkCol (no need to write it). Using your checkRow and checkCol methods as building blocks, write a method checkRowsCols that checks that all rows and columns are valid in the current board. (6 points)

Examples